This invention relates to a crucible for growing large macrocrystals (or ingots or boules) from a melt in a highly modified Stockbarger type furnace, or a comparable one in which slow growth of a solidifying macrocrystal from the melt under the influence of the earth's gravity, results in the solidified macrocrystal being supported within the walls of the crucible. An ideal macrocrystal is a single crystal, but typically it is only optically monocrystalline, being formed of several crystals demarcated by boundaries within the macrocrystal. Grain boundaries within a large macrocrystal (an ingot) which are discontinuities are undesirable.
More specifically the novel crucible is useful to grow crystals of halides of an element of Group 1a and Group 2a of the Periodic Table, particularly the alkali metal fluorides and alkaline earth metal fluorides, lead fluoride and crystals of the foregoing which are doped with desirable metal ion dopands. High quality macro-crystals of fluorides of lithium and sodium, as well as of magnesium, calcium, barium and strontium have been produced commercially since soon after 1939 when Donald Stockbarger disclosed a method for their manufacture in U.S. Pat. No. 2,149,076 and we taught methods for purifying melts in U.S. Pat. Nos. 2,498,186 and 2,550,173. However, producing high quality macrocrystals depends upon many factors, including having the skills required to duplicate successful runs. Success depends upon anticipating the need for minute adjustments in "power hours" (rate at which power is delivered to heat the melt), among others, all of which adjustments must be made before a probe inserted in the melt shows any indication of a change in the rate of growth.
To date, a graphite crucible may be used for growing a Stockbarger macrocrystal provided the graphite is not so porous as to allow the melt to leak through it. A crucible may have an inclined or flat bottom and either might be provided with a well for a "seed crystal" holder.
U.S. Pat. No. 5,911,824 teaches that a particular graphite crucible could not contain a melt of thallium iodide doped sodium iodide, NaI(Tl) (see col 4, lines 26-34). A NaI(Tl) crystal was successfully grown in a graphite crucible coated the inside surface of which is coated graphitic pyrolitic carbon, and the crystal did not adhere to the coated surface (see col 4, lines 36-47).
It is preferred to use a 9RL grade graphite manufactured by Airco Carbon after it is cleaned with a 44% HI solution, rinsed with deionized water, and heated to 400-500.degree. C. for a full day to dry it. It has a generally tapered trapezoidal or conical bottom, the width of the bottom being much less than that of the top of the crucible, primarily due to considerations of strength. It is a characteristic of macrocrystals of the aforementioned fluorides of melt-grown metal or metal-like elements, that they shrink when they solidify (referred to herein as "shrinking melts"), and the first crystal grown was readily removed because the walls were not wetted by the melt. However, a second crystal grown in the same crucible, adhered strongly to the crucible and could not be removed in its entirety without a remelting procedure (see col 4, lines 60-66). If the crystal was grown in a platinum crucible, the crystal adheres to the platinum. The platinum crucible must be heated to melt the surfaces of the crystal in contact with the platinum (referred to as "remelting") before the platinum crucible may be slid off the crystal.
To be readily removed, sufficient shrinkage must occur when the melt solidifies. Since the coefficient of linear expansion of graphite is about 7.85 microinches/(inch)(.degree. C.) measured at about 40.degree. C., that of the salt must be greater, preferably at least 5% greater, and more preferably 10% greater. The coefficient of fluorspar is about 19.5 microinches/(inch)(.degree. C.) measured at about 40.degree. C., so that even in a relatively small crucible having an inside diameter in the range from about 5 cm to about 25 cm, the circumferential surface of the solidified crystal pulls away from the inner walls of the crucible, and the crystal may be lifted out of the crucible with a vacuum cup without breaking the crystal or crucible, even when the crucible is cylindrical and its sides are vertical. This may be done provided the surface tension of the melt is high enough and does not wet the surface of the graphite, and not to seep into its pores. Note that the linear coefficients of expansion at melt temperature will likely be substantially different from those given above.
The effect of such shrinkage on a crystal no larger than about 7.5 cm at its greatest diameter (referred to as a 7.5 cm diameter macrocrystal) is not particularly notable even if the shrinkage is not controlled, but for larger crystals that effect is; the larger the crystal, the more detrimental are the effects of such uncontrolled shrinkage.
Using a shrinking melt to grow a Stockbarger macrocrystal from a melt typically comprises slowly moving the melt at a controlled rate from a region hotter than its solidification temperature to a region cooler than its solidification temperature, controlling the relative temperatures of the regions, and maintaining a temperature gradient in a localized zone between the regions at the boundary of the melt. The temperature gradient in the zone is sufficient to allow melt to crystallize at the cooler boundary of the localized zone. An "elevator" type furnace may be used where the crucible is raised or lowered on an elevator; or a "movable temperature gradient" furnace may be used where the furnace is moved and the crucible is stationary. The gradient between melt near the top of a crystal and the sharply localized zone is in the range from about 100.degree. C. to about 500.degree. C. depending upon the particular halide.
It will be evident that the temperature of the edge portion of successive layers of the melt corresponds to the solidification temperature of the melt as these edge portions reach a substantially fixed location in the path of travel of the melt and solidification begins and progresses inwardly. Preferred crystals are obtained when the zone of solidification approximates a plane. It is desirable to control the rate of heat flow through the inner portion of the melt from the hotter to the cooler region. If the rate of heat flow through the inner portion of the melt is too slow, the zone of solidification tends to be concave. Properly controlling the rate of heat flow through the inner portion of the melt allows the zone of solidification to approximate a plane.
To date, the art has addressed the problem of a macrocrystal adhering to the inner surface of the crucible by either providing a very smooth microporous graphite surface, or by coating the surface of the graphite as in the '824 patent, or by lining a mechanically stable temperature resistant material such as alundum or graphite with a thin sheet of platinum as in U.S. Pat. No. 5,997,640. In either case, the better is the separation upon solidification, the more readily the macrocrystal falls to the bottom of the crucible. In a crucible in which the porosity is such that the melt seeps into the pores for a short distance sufficient to hold the weight of the crystal, the bottom of the solidified crystal pulls away from the bottom of the crucible, leaving a bottom gap. In many instances the seepage is insufficient to support the weight of the crystal in the walls of the crucible and the crystal will suddenly drop to the bottom of the crucible closing the gap.